1. Field of the Invention
The present invention relates to a continuous process for the preparation of nitrophenols and, more especially, to the continuous preparation of nitrophenols by nitrating phenol in an aqueous medium.
2. Description of the Prior Art
The nitration of phenol is a reaction which has long been known to this art. Numerous publications exist on this subject and very frequently they are divergent with regard to the mechanism of the reaction or, more precisely, of the competing reactions (nitration, nitrosation, oxidation, dinitration and polycondensation). One of the basic problems that is encountered when phenol is nitrated is that of the selectivity of the nitration. Indeed, direct nitration of phenol produces an ortho-nitrophenol/para-nitrophenol ratio which is substantially 50/50. However, industrial demand for ortho-nitrophenol is much less than that for para-nitrophenol. As a result of this, to satisfy the industrial requirements for para-nitrophenol, ortho-nitrophenol is produced in quantities greatly in excess of the demand therefor.
Various solutions to the above problem have been proposed in order to promote production of the para derivative, or even for obtaining only the para derivative. It has for example been proposed to prepare para-nitrosophenol, first by nitrosation with the aid of nitrous acid, and then oxidizing the para-nitrosophenol to para-nitrophenol. Such a process is, economically, relatively unprofitable. Furthermore, nitrosophenol is an unstable compound which presents thermal explosion hazards. Therefore, it is preferable not to accumulate this compound in large quantities.
Patent GB 1,098,717 describes the nitration of phenol with nitric acid in aqueous medium, in the presence of nitrous acid and sulfuric acid, with specified concentrations of these various acids, especially high concentrations of sulfuric acid. According to the alternative embodiment of such process, there is either a precipitation of solids in the reactor, or, in order to avoid such precipitation, a very slow rate of introduction of the phenol. From an industrial standpoint, such a process presents major technical problems (high concentration of sulfuric acid, precipitation of a solid, low production efficiency).
To overcome these disadvantages, FR 2,144,395 describes a process entailing reacting phenol or derivative thereof in an aqueous mixture of nitric acid and nitrous acid at a temperature of -15.degree. C. to +15.degree. C., while maintaining a certain concentration of nitric acid and an overall nitric acid/phenol molar ratio of 4/1 to 5/1.
This process produces a pasty reaction mixture, which is heated upon completion of the addition of phenol in order to dissolve the solids or suspension. Therefore, it is indeed a two-step process, which is difficult to implement continuously on an industrial scale.